The installation of our two bathtubs was also accompanied by many discussions. It was important to me that the tub is slightly lower than the tiles to create an additional barrier so that water does not splash out or I don't have to clean half the bathroom when cleaning the tub. You can't see it, but the tub is 2.5mm lower than the tiles. For me, that's perfect. The mineral cast tub rests on a frame and is supported in the middle with XPS. Whether that meets the standard, I dare to doubt, but we were able to lay underfloor heating loops in the XPS, so our shower area is now heated and we can step onto a warm tub in winter.